Re: [Xen-devel] [PATCH] xen/arm: Implement domain_get_maximum_gpfn

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 02.07.14 at 11:41, <Ian.Campbell@xxxxxxxxxx> wrote:
> On Wed, 2014-07-02 at 10:37 +0100, Julien Grall wrote:
>> On 02/07/14 10:22, Ian Campbell wrote:
>> > Any reason why both arm and x86 can't just use a fixed scratch pfn for
>> > this temporary mapping? Both of them surely have spaces which they can
>> > guarantee won't overlap with anything.
>> 
>> This was the previous behavior until last November.
>> 
>> commit db062c28f30eb68d1b5d7a910445a0ba1136179a
>> Date:   Wed Nov 13 09:26:13 2013 +0100
>> 
>>     libxc: move temporary grant table mapping to end of memory
>>     
>>     In order to set up the grant table for HVM guests, libxc needs to map
>>     the grant table temporarily.  At the moment, it does this by adding the
>>     grant page to the HVM guest's p2m table in the MMIO hole (at gfn 
>> 0xFFFFE),
>>     then mapping that gfn, setting up the table, then unmapping the gfn and
>>     removing it from the p2m table.
>>     
>>     This breaks with PVH guests with 4G or more of ram, because there is
>>     no MMIO hole; so it ends up clobbering a valid RAM p2m entry, then
>>     leaving a "hole" when it removes the grant map from the p2m table.
>>     Since the guest thinks this is normal ram, when it maps it and tries
>>     to access the page, it crashes.
>>     
>>     This patch maps the page at max_gfn+1 instead.
>> 
>> I'm not sure what to do for x86, so I was planning to introduce a per-arch 
> hook to retrieve a scratch gpfn.
>> x86 would keep the current behavior, and ARM will use the GNTTAB space in 
> the layout.
> 
> Perhaps x86 could use some well known MMIO space, like the APIC at
> 0xfff????

Except that PVH has no LAPIC right now. Yet with the recent hole
punching patches I wonder whether "there is no MMIO hole" is actually
correct. Roger?

Jan


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